NATO is holding its largest military exercise in more than a decade, a move that's being viewed as a counter to Russian actions in Ukraine and Syria. All four branches of the U.S. armed forces are taking part.

Exercise Trident Juncture 2015 will involve 36,000 personnel from more than 30 nations — including the U.S. The exercise is taking place mainly in Spain, Portugal and Italy, but will include portions on the Atlantic Ocean and Mediterranean Sea, as well as in Norway, Germany, the Netherlands, Belgium and Canada. It's the largest NATO exercise since 2002.

Five thousand U.S. troops from all branches of service will participate in Trident Juncture 2015. The live portion of the exercise began Oct. 19 and will run through Nov. 6. NATO already concluded the "Command Post Exercise" portion on Oct. 16, designed to bolster coordination and cooperation among international military leaders.

"Exercises such as these help ensure the agility and readiness to execute real-world missions with other nations at a moment's notice and as a complex and dynamic global environment may dictate," said Lt. Col. Christopher Karns, Air Force spokesman. "Trident Juncture 15 provides an opportunity to work with Allies and partners while enhancing collective capabilities. At the same time, it signals a strong and collective commitment to ensuring regional stability."

"Exercises like this one … are vital for our continued security," said U.S. Ambassador and NATO Deputy Secretary-General Alexander Vershbow at Trident Juncture's opening ceremonies. "Within a rapidly changing security environment, with new challenges appearing almost daily, ensuring the readiness and the operational capability of our forces is more important than it has ever been."

The exercise will allow NATO allies to prepare for rapid deployment to address any crisis in or near Europe, Vershbow said.

"In the east, Russia has illegally annexed Crimea, continues to support the separatists in eastern Ukraine and now appears to have entered the war in Syria, firmly on the side of Assad," he said. "And to our south, from Syria to Libya, failed and failing states have opened the door to extremist and terrorist groups, eager to fill the vacuum."

Trident Juncture 2015 will include traditional exercises, like joint war-fighting and ballistic missile defense, but it's also focusing on countering newer threats.

Participants will work on defending against cyber attacks, countering propaganda from state-owned media and combating hybrid warfare that includes outside states supporting militant groups in another nation, according to a NATO news release.

It's the same type of tactics Russia has been accused of using in Crimea, Ukraine and Syria.

The exercise will also try to address how militaries and nations can deal with refugees and other humanitarian crises caused by war. It comes at a time when Europe is trying to provide aid for more than three million Syrian refugees, according to a U.N. estimate.

Trident Juncture will focus on NATO protecting and defending the fictional nation of "Lakuta" against aggression from a regional adversary, "Kamon."

"Rising political instability, ethnic tension, and persisting socioeconomic challenges are climaxed by a blatant invasion of one state's territory by another," says the official NATO scenario.

Trident Juncture 2015 will include over 140 aircraft, more than 60 ships and seven submarines, according to a NATO news release.

"Exercise Trident Juncture will demonstrate that NATO is ready and able to defend any ally against any threat in any form," Vershbow said. "It will demonstrate our ability to move quickly and decisively beyond our borders to protect our partners and our interests.  And that we can deal with everything from conventional military engagements to more subtle hybrid warfare techniques and propaganda."